Callier Effect
Created on : December 15, 2023 17:58
Denotation
The difference in contrast between images created by a photographic film with various illumination techniques is known as the Callier effect. It is not to be confused with the sharpness variation, which is likewise caused by variations in partial coherence.
Introduction
The apparent difference in density and micro-contrast seen when illumination on a silver-based negative is changed from parallel or collimated to diffused. Also seen when dust specks in the atmosphere are invisible in diffused light but clearly visible against a dark background when illuminated by the parallel rays of the sun. The callier effect is caused by different way light is scattered according to type of illumination.
Description
The Callier effect explains the increase in contrast of a silver-based negative when it is printed with a condenser enlarger compared to printing with a diffused light-source or a cold-cathode light. With colour materials, the diffuse and specular densities are about equal because there is little scattering by the dye-clouds; there is no practical change in contrast with different enlarger heads.
The Callier effect explains why condenser enlargers produce more contrast with conventional black and white materials than diffuser enlargers.
In black & white enlarging, contrast is relative to the amount of silver left on the negative.
The Callier effect is the change in image contrast produced by photographic film under different lighting methods. This should not be confused with sharpness changes due to partial coherence differences. Directional brightfield has very strong directional characteristics due to point sources and optics (condensers). In this case, each point on the photographic film receives light from only one direction. In the diffuse brightfield setup on the other hand, illumination of the film is provided through a translucent slab (diffuser), with each point on the film receiving light from a wide range of directions.
Illumination collimation in a film plays a fundamental role in the contrast of the image impressed on the film. At high scattering rates, the attenuation introduced by image particles varies significantly with the degree of illumination collimation.
The same silver-based film is reproduced with directed and diffuse brightfield setups. The overall contrast also changes. The contrast on the left is much stronger than the contrast on the right. In the absence of scattering, the attenuation introduced by the emulsion is independent of illumination collimation. Dense points absorb most of the light and less dense points absorb less, regardless of the directional characteristics of the incident light.